What You Need to Know About...High Purity Water

By William S. Siegmund

In 1987, then president of the United States, Ronald Reagan, hailed the success of the Montreal Protocol on substances that deplete the ozone layer. This protocol eventually led to limiting and, finally, an outright ban on chlorofluorocarbon containing chemicals (CFC’s). Chief among them was Freon, which was widely used as cleaning solvent, especially in critical cleaning application.

This gave rise to the use of water as the primary cleaning agent and Ultra Pure Water (UPW) for critical cleaning operations. To understand how water works, we will need to go back to the basics, discussing what water is, how to measure quality, treatment techniques and system designs to purify water, as well as practical applications in critical cleaning optical lens coating operations.

Water: A Most Unique Substance on Earth
Barren and lifeless is the only way to describe earth without life-giving water. Water can not be created or destroyed. Water is continually recycled from sky to earth to sky in a natural circuit called the Hydrologic Cycle. The Hydrologic Cycle has been called nature’s purification treatment. The earth is more than two-thirds water, but less than three percent is fresh and most of that is tied up (until recently) in polar ice caps or underground. We use water in so many ways that it is perhaps unrealistic to expect it to meet all the demands we make upon it. Still, with the right treatment, water can and does meet all of its obligations. There is no new water on earth, so it is easy to see how the demand of an ever-increasing population is contaminating this finite resource. Water is unique: it is the only substance on earth that at naturally occurring temperatures can exist in all states of matter: as ice it is a solid, we drink it, as a liquid, and as water vapor it is a gas. Water has a slightly blue color and does not conduct electricity (electrolyte), but instead it resists the flow of electrical current (resistor). “Pure water exists so rarely that practically speaking it is non-existent.”1

Water is the “universal solvent” and dissolves a little of everything it comes into contact with. Contaminants can include: a) dissolved minerals, b) dissolved gases, c) turbidity and sediment, d) color and organic matter, e) taste and odor and f) micro-organisms. Dissolved minerals are primarily salts (metals) that are dissolved in water. The water can be perfectly clear, but all naturally occurring water contains some degree of dissolved material measured as Total Dissolved Solids (TDS). There are different measures for each one of the impurities. You will see under treatment techniques and system design that we will show them all. I would like to limit our discussion to the most commonly referred to quality indicators used in optical laboratories.

How We Measure Quality

The most commonly used measurement for water quality relevant to critical cleaning are:

A. Parts per Million (ppm) is a measure of concentration, how many parts out of a million and can be expressed as milligram per liter mg/l.

B. Micro Siemens per Centimeter (µm/cm) is a measure of specific conductance or a material’s ability to conduct an electric current over a given distance (centimeter).

C. Mega ohms per Centimeter (mW/cm) are a measure of specific resistance or a material’s ability to impede the flow of electrical current over a given distance. 1 Meg ohm is equal to 1 million ohms of resistance.

Remember that water does not conduct electricity—it is a resistor. Also keep in mind that water is the universal solvent. What we are measuring is ionically charged particles dissolved in the water. As an example, if you were taking a bath in ultra pure water 10 mW/cm or better, and an electric fan fell in the tub, you would not get a shock. If you add a teaspoon of salt, you’re dead. (Please DO NOT try this at home!) Ionically charged particles are minerals (primarily salts) dissolved in water. They can be either (+) positively charged CATION or (-) negatively charged ANION. We are measuring their presence (specific conductance) or absence (specific resistance) as a measure of water quality.

The higher the specific conductivity, the lower the water quality. The higher the specific resistance, the higher the water quality.

High Specific Conductance = Low water quality

High Specific Resistance = Higher water quality

Treatment Technologies

Deionization, Reverse Osmosis, and Distillation are all demineralizing technologies. When we distill water, we use the fact that water can move from a liquid to a gas. By adding heat, we create a vapor and by cooling, we turn it back into a liquid. Only the dissolved minerals are left behind in the boiling chamber. Since few labs use distillation, we will move on to Reverse Osmosis.

Reverse Osmosis (RO) is also a demineralizing technology. To understand RO, it is helpful to understand osmosis. Osmosis is the movement of a solvent through a semi-permeable membrane from a lower to a higher concentration. The cells of our body will move needed nutrients through the cell wall (semi-permeable membrane) to supplement nutrients. Gortex is an example of a semi-permeable membrane. It contains a plastic layer with pores large enough to let vapor out, but too small to let liquid in.

With reverse osmosis, we use pressure to force a solvent-water with dissolved impurities through a semi-permeable membrane from a higher to a lower concentration. Under pressure, the water flows down into the pores of the membrane and works its way to the product water side. Charged ions (mineral salts) are repelled from the membrane surface and are transported away (rejected) with main flow of water (reject water). (See graphic below.) Reverse osmosis uses water to make water. The amount of water used versus the amount of product water produced is measured as a percentage called Percent Recovery. Newer designed systems can run from 50 to 75 percent recovery and do not “waste” water. In fact, some water is often recycled to the RO feed side or even used in other plant operations with little or no “waste.” RO as a demineralization technology is always a part of a system that combines treatment technologies, including micro filtration and carbon pre-treatment.

The dissolved minerals are attached to the resin beads and must be removed (regenerated) using, hydrochloric acid and sodium hydroxide. Since this is a highly toxic process, it is rarely or never done in the lab. Regenerated tanks are “exchanged” new for used.

Treatment Techniques, System Design

The treatment technique most commonly used in optical coating laboratories is Mixed Bed Deionization (DI). This system is usually applied using a carbon tank followed by two mixed bed (cation and anion) tanks in series because these tanks are only capable of producing water at 1 to 6 mW/cm. By using them in series, we can achieve the 10mW/cm (+) required for most coating applications. However, this is by far the highest cost per gallon.

There is a technology worth mentioning, electric-deionization (EDI). EDI uses electric current and resin to remove dissolved mineral salts. It is becoming cost viable for small systems, but requires careful pre-treatment controls that are more cost effective on larger systems.

The most cost effective way to produce High Purity Water (HPW) is a combination of a reverse osmosis system and mixed bed DI. The RO has the advantage of removing both dissolved and un-dissolved (NON-ionically charged) contaminants. By removing both, you improve the overall quality of the product water and increase the “life” of the mixed bed DI by as much as 70 percent.

Practical Applications

A properly designed high purity water system can reduce the cost per gallon while improving water quality and, therefore, yield. It must be integrated into the operation and contain adequate monitoring, as well as alarms to “baseline” the quality to prevent fluctuation. Lab operators can count on the quality water being delivered to the washer/coater with little programmed maintenance. This can also extend the life of any re-circulating “on board” resins as well.

In a recent industry article, “Anti-reflection Coating Quality,” the author lists “control of all process variables that can be automatically controlled and alarmed,” as one of several factors in “quality creation.”2

The importance of proper cleaning and rinsing of a lens prior to coating can not be over stated, and therefore the importance of proper High Purity Water Quality, as a controlled, alarmed variable, must not be overlooked.

William S Siegmund, CWS-V, is founder and president of Pure Water Works, Inc., an independent dealership with its international headquarters in Traverse City, Michigan. Mr. Siegmund has pioneered standards and developments for the water industry. www.purewaterworks.biz